An homologous radioimmunoassay for human relaxin will be established and its specificity studied prior to its application for measurement of relaxin in the human during the reproductive cycle. Uterine and testicular relaxins from the pig will be finally characterized. The pig and guinea pig will be used for in vivo studies on the control of relaxin secretion. The effect of different physiological states as well as estrogen priming on the rat myometrial relaxin receptor is to be studied. This work is to be extended in order to assess concomitantly relaxin receptors in the uterus and cervix in the pig and human. Chemically modified natural relaxin as well as synthetic relaxins will be studied in both bio and immuno assay systems in order to determine their structure-activity relationships.